The present invention relates generally to a technique for maintaining an effective X-ray dosage during imaging procedures. More particularly, the invention relates to increasing the X-ray dosage per image exposure while reducing the number exposure events such that an effective X-ray dosage is maintained during imaging.
Digital X-ray imaging systems are becoming increasingly widespread for producing digital data which can be reconstructed into useful radiographic images. In current digital X-ray imaging systems, radiation from a source is directed toward a subject, typically a patient in a medical diagnostic application. A portion of the radiation passes through the patient and impacts a detector. The surface of the detector converts the radiation to light photons which are sensed. The detector is divided into a matrix of discrete picture elements or pixels, and encodes output signals based upon the quantity or intensity of the radiation impacting each pixel region. Because the radiation intensity is altered as the radiation passes through the patient, the images reconstructed based upon the output signals provide a projection of the patient's tissues similar to those available through conventional photographic film techniques.
Digital X-ray imaging systems are particularly useful due to their ability to collect digital data which can be reconstructed into the images required by radiologists and diagnosing physicians, and stored digitally or archived until needed. In conventional film-based radiography techniques, actual films were prepared, exposed, developed and stored for use by the radiologist. While the films provide an excellent diagnostic tool, particularly due to their ability to capture significant anatomical detail, they are inherently difficult to transmit between locations, such as from an imaging facility or department to various physician locations. The digital data produced by direct digital X-ray systems, on the other hand, can be processed and enhanced, stored, transmitted via networks, and used to reconstruct images which can be displayed on monitors and other soft copy displays at any desired location. Similar advantages are offered by digitizing systems which convert conventional radiographic images from film to digital data.
Despite their utility in capturing, storing and transmitting image data, digital X-ray systems are still overcoming a number of challenges. For example, X-ray systems may be employed for a range of different types of examination, including radiographic and fluoroscopic imaging. Among other distinctions, these two types of imaging examinations are characterized by significantly different radiation levels used to generate the image data. Specifically, radiographic imaging sequences employ substantially higher radiation levels than fluoroscopic imaging sequences. In a number of applications, it may be desirable to perform both types of imaging sequences sequentially to obtain different types of data and to subject patients to lower overall radiation levels. However, current digital X-ray systems may encounter difficulties in performing fluoroscopic imaging sequences following radiological sequences.
Specifically, current digital X-ray systems employ amorphous silicon detectors with arrays of photodiodes and thin film transistors beneath an X-ray scintillator. Incident X-rays interact with the scintillator to emit light photons which are absorbed by the photodiodes, creating electron-hole pairs. The diodes, which are initially charged with several volts of reverse bias, are thereby discharged in proportion to the intensity of the X-ray illumination. The thin film transistor switches associated with the diodes are then activated sequentially, and the diodes are recharged through charge sensitive circuitry, with the charge needed for this process being measured.
Raw signals from the detector may require several corrections to yield an accurate measure of the incident X-ray intensity. One of these corrections is for offset, or the signal which exists in the absence of X-ray illumination, which may result from current leakage in the diodes.
A second source for this residual signal is the previous history of illumination of the diodes, a phenomenon known as lag. Lag occurs when the signal strength associated with a pixel depends upon the preceding X-ray exposure event or events. Due to the nature of the amorphous silicon of the detector panel, the photodiodes contain traps which are filled after X-ray excitation, and which thereafter empty in a decay process with a relatively long time constant. As a result, a decaying image is retained by the detector. The magnitude of image retention in X-ray detectors is relatively small, and decays with time as the traps empty thermally so that the lag signal will slowly decay away until it is no longer visible. In single-shot radiographic applications, image retention does not generally cause problems because a relatively long period of time exists between exposures.
Image retention in X-ray detectors poses a substantial problem, however, in applications requiring mixed radiographic and fluoroscopic operation. Again, because the fluoroscopic signal levels are substantially lower (e.g. two to three orders of magnitude smaller) than the radiographic signals, when a fluoroscopic imaging sequence follows a radiographic exposure, the retained image, although a small fraction of the radiographic signal, can be comparable to or even larger than the fluoroscopic signal. If uncorrected, a ghost of the radiographic image will appear in the reconstructed fluoroscopic image.
One technique which is employed to reduce the effects of lag in mixed radiographic and fluoroscopic operation is to operate the X-ray tube at half the frame rate of the X-ray detector during fluoroscopy. Because of this differential between the operation of the X-ray tube and the detector, every alternate reading of the detector occurs in the absence of an X-ray exposure and therefore provides a measure of lag at that acquisition time. The measures of lag determined from these dark frames can then be used to correct the light frames either in real-time or offline.
However to maintain the same image quality while employing this lag correction technique, approximately the same dose of X-rays per second must be delivered to the detector with half the number of exposures. To accomplish this dosage requirement, either the X-ray flux per exposure or the duration of the exposure is doubled or an equivalent combination of increased flux and duration is employed. In some operating conditions, such as thick patients, the maximum duration may already be employed, however. Likewise increasing the peak X-ray flux per exposure can stress the X-ray tube and thereby degrade the tube lifetime.
There is a need, therefore, for an improved technique for maintaining image quality while allowing for lag correction during mixed radiographic and fluoroscopic operations. There is a particular need for a technique which can increase the available exposure time during fluoroscopic imaging such that the same X-ray dose per second can be delivered to the detector at a reduced number of exposures without degrading X-ray tube performance.